Hydrostatic transmissions have many uses, including the propelling of vehicles, such as mowing machines. A typical hydrostatic transmission system includes a variable displacement hydraulic pump connected in a closed hydraulic circuit with a fixed or variable displacement hydraulic motor. The closed hydraulic circuit includes a first conduit connecting the main pump outlet with the motor inlet and a second conduit connecting the motor outlet with a pump inlet. For most applications, the pump is driven by a prime mover, such as an internal combustion engine or an electrical motor, at a certain speed in a certain direction. Changing the displacement of the main pump will change its output flow rate, which controls the speed of the motor. Pump outflow can be reversed, thus reversing the direction of the motor. In a vehicle, the motor is often connected through suitable driving means to the vehicle's wheels or tracks, such as through gearing and/or coupling to an output shaft. In some vehicles, such as zero-turn-radius mowers, separate hydrostatic transmissions are used to independently drive separate wheels of the vehicle.
The hydraulic pump usually is a piston-type pump including a plurality of reciprocating pistons sliding axially within a barrel which are in fluid communication through hydraulic porting with the hydraulic motor. Rotation of the barrel and driving of the pistons against a rotatable swash plate creates an axial motion of the pump pistons that forces hydraulic fluid through the hydraulic porting to the hydraulic motor to drive the motor. The rotated angle of the swash plate with respect to the hydraulic pump pistons can be changed to vary the speed and change the direction of rotation of the hydraulic motor and thus the speed and direction of rotation of the vehicle axle. The orientation with which the swash plate addresses the hydraulic pump pistons can be changed to control whether the hydraulic motor rotates in the forward direction or in the reverse direction. Additionally, the angle at which the swash plate addresses the hydraulic pump pistons can be changed to increase or decrease the amount of hydraulic fluid that is forced from the hydraulic pump to the hydraulic motor to change the speed at which the hydraulic motor rotates.
The position of the rotatable swash plate typically is varied by rotating a trunnion arm that is coupled to the swash plate. The trunnion arm projects from the pump housing for coupling to a speed change lever or a speed change pedal of the vehicle through a cable, lever or other linkage. In this manner, movement of the speed change lever/pedal results in rotational movement of the trunnion arm to change the position of the swash plate to thereby control the speed and direction of the vehicle.
Return to neutral mechanisms typically have been used to cause the swash plate of the pump to assume a neutral position after having been shifted to a forward or reverse position. These return to neutral mechanisms are typically made of multiple parts and may be difficult to assemble, or are required to be integrated into the vehicle linkage, and thus generally do not permit sufficient adjustment of the return to neutral mechanism during assembly to adequately set the neutral position, while also minimizing lag in movements between the mechanism components when they are moved toward forward or reverse positions.